Remove .gitignore

A lot of the .gitignore build was either outdated (for unused software,
or for the time we didn't have separate build & source directories), or
really should be something people shuld be putting in either their own
global `.gitignore`, or in `.git/info/exclude` for a local set of files
git should ignore.

This also removes the annoying problem that we can't easily add or
update .patch files we need to add for the flatpak build once in a
while.

.gitignore

@@ -1,28 +0,0 @@
-*~
-.valencia
-/.stamp
-build/
-_build/
-debian/*.debhelper.log
-debian/*.substvars
-debian/.debhelper
-debian/debhelper-build-stamp
-debian/files
-debian/geary
-debian/geary-dbg
-/geary
-/geary-mailer
-/geary-console
-.flatpak-builder
-*.diff
-*.patch
-*.xz
-*.swp
-__pycache__
-bindings/vapi/gmime-2.6/gmime-2.6.gi
-/valadoc
-/po/untitled.pot
-/help/*/*.page
-!/help/C/*.page
-src/sqlite3-unicodesn/unicodesn.sqlext
-cbellega.json