Remove check for APPLICATIVE_IN_BASE cpp def

As we currently depend on base == 4.*, there's no need to check whether
Control.Applicative is provided in base -- it has been since base 2.x.

Furthermore, as far as I can tell, the APPLICATIVE_IN_BASE preprocessor
variable wasn't even being set anywhere, so the #ifdef directives
actually hid the code blocks inside them in all cases, not only when
base was sufficiently new.

This commit makes Applicative-Monad proposal -related warnings from GHC
7.8 go away.