gitignore: clean up patterns from old times

The gitignore file currently has a lot of patterns for files that we
shouldn't write anymore since we have migrated to CMake, as everybody is
expected to do out-of-source builds anyway. Let's remove them.

.gitignore

-/tests/clar.suite
-/tests/clar.suite.rule
-/tests/.clarcache
-/apidocs
-/trash-*.exe
-/libgit2.pc
-/config.mak
-*.o
-*.a
-*.exe
-*.gcda
-*.gcno
-*.gcov
-.lock-wafbuild
-.waf*
 build/
-build-amiga/
-tests/tmp/
-msvc/Debug/
-msvc/Release/
-*.sln
-*.suo
-*.vc*proj*
-*.sdf
-*.opensdf
-*.aps
-*.cmake
-!cmake/Modules/*.cmake
 .DS_Store
 *~
 .*.swp
 tags
-mkmf.log
-*.profdata
-*.profraw
 CMakeSettings.json
 .vs