environment.h: remove unused variables

environment.h: remove unused variables

packed_git_window_size and packed_git_limit are not used anywhere in
the codebase. A search found that all references were removed in
d284713bae (config: make `packed_git_(limit|window_size)` non-global
variables, 2024-12-03), except the ones in this file, as they were moved
to struct repo_settings.

Remove packed_git_window_size and packed_git_limit from environment.h.

Signed-off-by: Arnav Bhate <redacted>
Acked-by: Patrick Steinhardt <redacted>
Signed-off-by: Junio C Hamano <redacted>