Revert "gtksourceview3-no-gi/ext/gtksourceview3/ -> gtksourceview3-no-gi/ext/gtksourceview3-no-gi/"

This reverts commit 3ece8575e52a6990835ee7f52bcc37df85113aa4.

Because gtksourceview3-no-gi/ext/gtksourceview3-no-gi/gtksourceview3/ is
strange.