[SDK:CMAKE] Remove unused BUILD_MP build option (reactos#7862)

It was introduced in commit 1f9c494 (r38270), but this flag isn't used
anymore in our source tree, and nowadays, we actively compile both UP
and MP support.

Author: HBelusca

sdk/cmake/config.cmake

@@ -92,8 +92,6 @@ else()
     set(_WINKD_ FALSE CACHE BOOL "Whether to compile with the KD protocol.")
 endif()
 
-option(BUILD_MP "Whether to build the multiprocessor versions of NTOSKRNL and HAL." ON)
-
 cmake_dependent_option(ISAPNP_ENABLE "Whether to enable the ISA PnP support." ON
                        "ARCH STREQUAL i386 AND NOT SARCH STREQUAL xbox" OFF)