depends: remove FORCE_USE_SYSTEM_CLANG

depends/README.md

@@ -119,9 +119,6 @@ The following can be set when running make: `make FOO=bar`
 - `DEBUG`: Disable some optimizations and enable more runtime checking
 - `HOST_ID_SALT`: Optional salt to use when generating host package ids
 - `BUILD_ID_SALT`: Optional salt to use when generating build package ids
-- `FORCE_USE_SYSTEM_CLANG`: (EXPERTS ONLY) When cross-compiling for macOS, use Clang found in the
-  system's `$PATH` rather than the default prebuilt release of Clang
-  from llvm.org. Clang 8 or later is required
 - `LOG`: Use file-based logging for individual packages. During a package build its log file
   resides in the `depends` directory, and the log file is printed out automatically in case
   of build error. After successful build log files are moved along with package archives