read-cache: remove redundant extern declarations

read-cache: remove redundant extern declarations

The 'read-cache.c' file already includes 'environment.h', which provides
the extern declarations for variables like 'trust_executable_bit' and
'has_symlinks'.

Remove the redundant extern declarations inside 'st_mode_from_ce()' to
clean up the code.

Mentored-by: Christian Couder <redacted>
Mentored-by: Ayush Chandekar <redacted>
Mentored-by: Olamide Caleb Bello <redacted>
Signed-off-by: Tian Yuchen <redacted>
Signed-off-by: Junio C Hamano <redacted>