OvmfPkg/README: Remove UNIXGCC

OvmfPkg/README: Remove UNIXGCC

Remove UNIXGCC in OvmfPkgIa32.dsc, OvmfPkgIa32X64.dsc
and OvmfPkgX64.dsc.
Remove content related to UNIXGCC in README.
https://bugzilla.tianocore.org/show_bug.cgi?id=1377

Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Anthony Perard <anthony.perard@citrix.com>
Cc: Julien Grall <julien.grall@linaro.org>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Shenglei Zhang <shenglei.zhang@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>