Remove gpl_source.tgz

This file was an archive of all the gpl files that could be found
in the source tree. Because it looks for gpl files using $(wildcard),
kati had to rerun the wildcards every single build to see if they had
changed. These GPL wildcards made up 30294 of 63996 wildcards present
on aosp-master. Removing these wildcards saves (aosp/internal) ~0.2/~0.1
seconds from every build, and ~0.4/~0.6 seconds from the wildcard
checking portion of kati. (The whole build sees <0.4s/0.6s of
improvement because it's parallelized with the shell checking, but
improvements here mean that we can now improve the shell commands as
well and get more fruitful results)

We don't actually use gpl_source.tgz anywhere, so we can just remove it.

Bug: 282079550
Test: m nothing
Change-Id: I77141dbb39b2d8ebd96bc258fe7dbdff5a372977