Remove constant conditionals (dead-code).

Some variables are set and never changed, making them constant.  This
means that code in the 'false' block of the conditional is unreachable.

In chan_skinny and res_config_ldap I used preprocessor directive `#if 0`
as I'm unsure if the unreachable code could be enabled in the future.

Change-Id: I62e2aac353d739fb3c983cf768933120f5fba059