[lldb/unittests/CMakeLists.txt] Remove extra compiler flag `-include gtest_common.h`, NFC

[akyrtzi]

Summary

This doesn't seem to be necessary anymore so remove it to be more consistent with rest of the LLVM projects
that don't use prefix headers.

[JDevlieghere]

Cool

lldb/unittests/CMakeLists.txt

 add_custom_target(LLDBUnitTests)
 set_target_properties(LLDBUnitTests PROPERTIES FOLDER "lldb tests")
 
 add_dependencies(lldb-unit-test-deps LLDBUnitTests)
 
 include_directories(${LLDB_SOURCE_ROOT})
 include_directories(${LLDB_PROJECT_ROOT}/unittests)
 
 if (CXX_SUPPORTS_SUGGEST_OVERRIDE_FLAG)
   add_compile_options("-Wno-suggest-override")
 endif()
 
-set(LLDB_GTEST_COMMON_INCLUDE ${CMAKE_CURRENT_SOURCE_DIR}/gtest_common.h)
-if (MSVC)
-  list(APPEND LLVM_COMPILE_FLAGS /FI ${LLDB_GTEST_COMMON_INCLUDE})
-else ()
-  list(APPEND LLVM_COMPILE_FLAGS -include ${LLDB_GTEST_COMMON_INCLUDE})
-endif ()
-
 function(add_lldb_unittest test_name)
   cmake_parse_arguments(ARG
     ""
     ""
     "LINK_LIBS;LINK_COMPONENTS"
     ${ARGN})
 
   if (NOT ${test_name} MATCHES "Tests$")