Vlv2TbltDevicePkg: Clean up the unused variables and function

In PlatformBootOption.c:

* mContinueBoot and mBootMenuBoot were assigned as "!EnterSetup" before
  being used. However, EnterSetup is always "FALSE", so it's meaningless
  to keep those variables.

* mPxeBoot is defined but never used.

* DetectKeypressCallback() is never used. So we can remove it altogether
  with mHotKeypressed and HotKeyEvent.

* RegisterBootOptionHotkey() only uses EFI_INPUT_KEY from EFI_KEY_DATA,
  so we don't need to use EFI_KEY_DATA for F2 and F7 in
  RegisterStaticHotkey().

v2:
  Update the description for the removal of EFI_KEY_DATA

Cc: Zailiang Sun <zailiang.sun@intel.com>
Cc: Yi Qian <yi.qian@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Signed-off-by: Gary Lin <glin@suse.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>