rust: kbuild: remove unused variable

Since we are adding one more proc macro crate (`zerocopy-derive`),
we are refactoring their handling.

`libpin_init_internal_extension` was added to mimic the setup for
`macros`, but it is not used, since the extension is expected to be
the same.

Thus remove it.

Reviewed-by: Nicolas Schier <nsc@kernel.org>
Link: https://patch.msgid.link/20260608141439.182634-4-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

0 files changed, 0 insertions, 0 deletions