Remove unused custom help.css file

The file is no longer used and was removed upstream for 4.0:
https://bugzilla.mozilla.org/show_bug.cgi?id=591535#c40

Grep'ing upstream and our custom changes for the two definitions
in that file also lead to zero results.

Bug: 54823
Change-Id: I2606bbd11b0e0c77b84775e3e6bb78b21a01b162
Merge "Remove unused custom help.css file"