build: Remove */.gitignore files

Since out-of-source-tree builds are now used after switching to meson,
we don't need .gitignore files in the source directories to ignore
build artifacts.

This fixes build errors when doing a meson build after an autotools
build, because generated files such as gio/xdp-dbus.c won't show up in
a `git status`, or be removed by a `git clean -f`, and so it won't be
obvious that such files need to be removed for the meson build to
succeed.

tests/.gitignore

@@ -1,76 +0,0 @@
-assert-msg-test
-asyncqueue-test
-atomic-test
-base64-test
-bit-test
-bookmarkfile-test
-child-test
-closures
-collate.out
-completion-test
-convert-test
-cxx-test
-datetime
-deftype
-dirname-test
-env-test
-file-test
-file-test-get-contents
-gio-test
-iochannel-test
-iochannel-test-outfile
-list-test
-mainloop-test
-mapchild
-mapping-test
-maptest
-markup-collect
-markup-escape-test
-markup-test
-module-test
-objects
-objects2
-onceinit
-patterntest
-properties
-properties2
-properties3
-properties4
-qsort-test
-queue-test
-regex-test
-relation-test
-scannerapi
-sequence-test
-shell-test
-signal1
-signal2
-signal3
-signal4
-slice-color
-slice-concurrent
-slice-test
-slice-threadinit
-slist-test
-sources
-spawn-test
-testgdate
-testgdateparser
-testglib
-testgobject
-testingbase64
-testmarshal.c
-testmarshal.h
-thread-test
-threadpool-test
-timeloop
-timeloop-closure
-tree-test
-type-test
-unicode-caseconv
-unicode-collate
-unicode-encoding
-unicode-normalize
-uri-test
-utf8-pointer
-utf8-validate