MdeModulePkg DxeCore: Don't cache memory mapped IO FV.

Previous DxeCore FwVol code will cache whole FvMain FV from flash that may be
uncached if platform reports FvMain FVB, it will impact DXE performance.

The code already has file level cache, so don’t need to cache memory mapped IO FV.
It can also reduce memory consumption of caching memory mapped IO FVs.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@15916 6f19259b-4bc3-4df7-8a09-765794883524