[Frontend] Delete a redundant check of -pg for setFramePointer()

Driver errors if -fomit-frame-pointer is used together with -pg.
useFramePointerForTargetByDefault() returns true if -pg is specified.
=>
(!OmitFP && useFramePointerForTargetByDefault(Args, Triple)) is true
=>
We cannot get FramePointerKind::None