llvm: remove redundant CMake variables

This recipe uses the cmake class, so there's no need to set the Ninja
backend, or tell it where Python is, or that we're cross-compiling.

(From OE-Core rev: 2560665cc1e30a6ca969118ade8612393e1eef74)

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-devtools/llvm/llvm_git.bb

@@ -82,9 +82,8 @@ EXTRA_OECMAKE += "-DLLVM_ENABLE_ASSERTIONS=OFF \
                   -DLLVM_OPTIMIZED_TABLEGEN=ON \
                   -DLLVM_TARGETS_TO_BUILD='${LLVM_TARGETS}' \
                   -DLLVM_TEMPORARILY_ALLOW_OLD_TOOLCHAIN=ON \
-                  -DPYTHON_EXECUTABLE=${HOSTTOOLS_DIR}/python3 \
                   -DCMAKE_BUILD_TYPE=Release \
-                  -G Ninja"
+                 "
 
 EXTRA_OECMAKE:append:class-target = "\
                   -DCMAKE_CROSSCOMPILING:BOOL=ON \