kbuild: remove unnecessary export of RUST_LIB_SRC

kbuild: remove unnecessary export of RUST_LIB_SRC

If RUST_LIB_SRC is defined in the top-level Makefile (via an environment
variable or command line), it is already exported.

The only situation where it is defined but not exported is when the
top-level Makefile is wrapped by another Makefile (e.g., GNUmakefile).
I cannot think of any other use cases.

I know some people use this tip to define custom variables. However,
even in that case, you can export it directly in the wrapper Makefile.

Example GNUmakefile:

    export RUST_LIB_SRC = /path/to/your/sysroot/lib/rustlib/src/rust/library
    include Makefile

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>