Remove conditional compilation (_AFNETWORKING_PIN_SSL_CERTIFICATES_)

This pull request removes the _AFNETWORKING_PIN_SSL_CERTIFICATES_ condition.

Originally, conditional compilation was used to sidestep the fact that if

`connection:willSendRequestForAuthenticationChallenge:`

is implemented, then

`connection:canAuthenticateAgainstProtectionSpace:`,
`connection:didReceiveAuthenticationChallenge:`
`connectionShouldUseCredentialStorage:`

would not be called. In the first draft of SSL pinning, *connection:willSendRequestForAuthenticationChallenge:* only supported the pinned use case. Now that *connection:willSendRequestForAuthenticationChallenge:* includes a case for AFSSLPinningModeNone, the implementations of *connection:willSendRequestForAuthenticationChallenge:* and *connection:canAuthenticateAgainstProtectionSpace:* are no longer necessary for the unpinned use case.