Consider removing is_user_valid macro (small optimization)

[rivit98]

is_user_alive and is_user_connected natives are checking range of id, so additional check is not necessary (is_user_valid)

• is_user_connected amxmodx.cpp
• is_user_alive amxmodx.cpp

Occurences of is_user_valid https://github.com/TheDoctor0/CSGOMod/search?q=is_user_valid

[rivit98]

Thanks!