pidgin/purple-plugin-pack

Remove plugin_pack.spec.in as we haven't provided rpms in a long time

Testing Done:
Ran `ninja dist` and verified no issues.

Reviewed at https://reviews.imfreedom.org/r/2404/
syntax: glob
*.gmo
.*.swp
syntax: regexp
.+\.spec
.*\.dll
buddytime/gtktimezonetest
buddytime/recursetest
build.*/
po/missing
po/notexist
po/POTFILES$
po/stamp-it
po/.+\.pot
purple-plugin-pack-\d+.+
win32-dist