builtin/receive-pack: drop redundant tmpdir env

builtin/receive-pack: drop redundant tmpdir env

When performing the connectivity checks for a shallow ref in
`update_shallow_ref()`, the child process environment variables are
populated via `tmp_objdir_env()`. This is unnecessary though as
`update_shallow_ref()` is only reached after `tmp_objdir_migrate()` has
been performed which means there is no longer a temporary directory that
needs to be shared with child processes.

Drop the call to `tmp_objdir_env()` accordingly.

Signed-off-by: Justin Tobler <redacted>
Signed-off-by: Junio C Hamano <redacted>