Maintainers.txt: Drop deprecated SourceForge SVN link

Maintainers.txt: Drop deprecated SourceForge SVN link

The commit will remove the deprecated edk2 SVN mirror at:
https://svn.code.sf.net/p/edk2/code/trunk/edk2

After this commit, the above mirror will not be synced with the changes
made in the GitHub upstream.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Andrew Fish <afish@apple.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Signed-off-by: Hao A Wu <hao.a.wu@intel.com>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>