Remove $g_cookie_secure_flag_enabled global var

With the introduction of http_is_https_protocol() function [1], this
global variable is not really needed anymore, as we can simply call
the function whenever we need to set a cookie's secure flag.

Fixes 0037007 (PR https://github.com/mantisbt/mantisbt/pull/2202)

[1]: see commit f39ad8c9816fefd36747630d25b7355265eaacc5