edk2: Update ReadMe.md and Maintainer.txt for edk2-libc repo

edk2: Update ReadMe.md and Maintainer.txt for edk2-libc repo

https://bugzilla.tianocore.org/show_bug.cgi?id=1734

The following packages have been moved to the new edk2-libc
repository.  Remove references to these packages from
Readme.md and Maintainers.txt.

* AppPkg
* StdLib
* StdLibPrivateInternalFiles

Cc: Jaben Carsey <jaben.carsey@intel.com>
Cc: Daryl McDaniel <edk2-lists@mc2research.org>
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>