[CMake] Remove set(CACHE) line for SWIFT_MODULE_ABI_NAME_PREFIX

Due to https://cmake.org/cmake/help/latest/policy/CMP0126.html
set(CACHE) overwrites the existing value if the cache has not been set
to any value. Since we don't need any default value, just remove it.

Author: rintaro

CMakeLists.txt

@@ -22,8 +22,6 @@ if(CMAKE_VERSION VERSION_LESS 3.21)
   endif()
 endif()
 
-set(SWIFT_MODULE_ABI_NAME_PREFIX CACHE STRING "ABI name prefix to avoid name conflicts")
-
 # The subdirectory into which host libraries will be installed.
 set(SWIFT_HOST_LIBRARIES_SUBDIRECTORY "swift/host")