Remove npm package from Ubuntu installation dependencies file.

Install of edX gives the following error:
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The following packages have unmet dependencies:

nodejs : Conflicts: npm

E: Unable to correct problems, you have held broken packages.
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This is caused by the latest version of nodejs including npm and being
incompatible with the previous version.

Fix is to remove npm line from apt-packages.txt

apt-packages.txt

@@ -22,5 +22,4 @@ libreadline6
 libreadline6-dev
 mongodb
 nodejs
-npm
 coffeescript