MB-69750: Remove unused valgrind subdirectory

These files are not referenced anywhere in our source code

Change-Id: I573b48fd8a5b16797cbcdb6dc48d140a8463b7b1
Reviewed-on: https://review.couchbase.org/c/platform/+/237429
Tested-by: Build Bot <build@couchbase.com>
Reviewed-by: Mohammad Zaeem <mohammad.zaeem@couchbase.com>

Author: trondn

CMakeLists.txt

@@ -109,7 +109,6 @@ CONFIGURE_FILE (${CMAKE_CURRENT_SOURCE_DIR}/include/platform/dynamic.in.h
 #
 # Set the include path
 #
-include_directories(BEFORE SYSTEM ${CMAKE_CURRENT_SOURCE_DIR}/external)
 include_directories(AFTER SYSTEM ${gsl_lite_SOURCE_DIR}/include)
 
 IF (MEMORY_ALLOCATOR)