Remove unused scss files.

We have some Sass files that basically generate duplicates of other
Sass files and are no longer referenced from any pages. This looks
like it might have been stuff that never got cleaned up when
comprehensive themeing was done.

List of removed files and what they were replaced by:

    application.scss ->  lms-main.scss
    application-rtl.scss -> lms-main-rtl.scss
    course.scss -> lms-course.scss
    course-rtl.scss -> lms-course-rtl.scss

This should help speed up the Sass compile.

lms/static/sass/application-rtl.scss

-/*
- * This file is dynamically generated and ignored by Git.
- * DO NOT MAKE CHANGES HERE. Instead, go edit its template:
- * /edx/app/edxapp/edx-platform/lms/static/sass/application-rtl.scss.mako
- */
-// lms - css application architecture
-// ====================
-
-// libs and resets *do not edit*
-@import 'bourbon/bourbon'; // lib - bourbon
-@import 'vendor/bi-app/bi-app-rtl'; // set the layout for right to left languages
-
-// BASE  *default edX offerings*
-// ====================
-
-// base - utilities
-@import 'base/reset';
-@import 'base/variables';
-@import 'base/mixins';
-
-
-@import 'build-lms'; // shared app style assets/rendering
-

lms/static/sass/application.scss

-/*
- * This file is dynamically generated and ignored by Git.
- * DO NOT MAKE CHANGES HERE. Instead, go edit its template:
- * /edx/app/edxapp/edx-platform/lms/static/sass/application.scss.mako
- */
-// lms - css application architecture
-// ====================
-
-// libs and resets *do not edit*
-@import 'bourbon/bourbon'; // lib - bourbon
-@import 'vendor/bi-app/bi-app-ltr'; // set the layout for left to right languages
-
-// BASE  *default edX offerings*
-// ====================
-
-// base - utilities
-@import 'base/reset';
-@import 'base/variables';
-@import 'base/mixins';
-
-
-@import 'build-lms'; // shared app style assets/rendering

lms/static/sass/course-rtl.scss

-/*
- * This file is dynamically generated and ignored by Git.
- * DO NOT MAKE CHANGES HERE. Instead, go edit its template:
- * /edx/app/edxapp/edx-platform/lms/static/sass/course-rtl.scss.mako
- */
-@import 'bourbon/bourbon';
-@import 'vendor/bi-app/bi-app-rtl'; // set the layout for right to left languages
-
-@import 'base/reset';
-@import 'base/variables';
-@import 'base/font_face';
-@import 'base/mixins';
-
-
-@import 'build-course'; // shared app style assets/rendering

lms/static/sass/course.scss

-/*
- * This file is dynamically generated and ignored by Git.
- * DO NOT MAKE CHANGES HERE. Instead, go edit its template:
- * /edx/app/edxapp/edx-platform/lms/static/sass/course.scss.mako
- */
-@import 'bourbon/bourbon';
-@import 'vendor/bi-app/bi-app-ltr'; // set the layout for left to right languages
-
-@import 'base/reset';
-@import 'base/variables';
-@import 'base/font_face';
-@import 'base/mixins';
-
-
-@import 'build-course'; // shared app style assets/rendering