Remove fb-sdk from base.txt requirements

Facebook wants from edX to upgrade all graphAPI calls to v2.9,
after investigation it can be seen that edX-platform is accessing
graphAPI through python-social-auth package that is already accessing
v2.9.'base.txt' of the requirements has fb-sdk package that is also used
to access graphAPI,but currently, there is no area in our code that is
using this library that's why there is no need of it in requirements.

LEARNER-2620

requirements/edx/base.txt

@@ -57,7 +57,6 @@ edx-search==1.1.0
 edx-submissions==2.0.12
 edxval==0.1.1
 event-tracking==0.2.4
-facebook-sdk==0.4.0
 feedparser==5.1.3
 firebase-token-generator==1.3.2
 GitPython==0.3.2.RC1