Further cleanup allowed by FloatingPoint implying Magnitude == Self

We no longer need to have a separate `abs` defined on FloatingPoint; we can use the existing function defined on `SignedNumeric` instead. Additionally mark `fabs` deprecated in favor of the Swift name `abs`; we don't need to have two names for the same function.

Author: stephentyrone

stdlib/public/Platform/tgmath.swift.gyb

@@ -14,8 +14,8 @@ import SwiftShims
 
 // Generic functions implementable directly on FloatingPoint.
 @_transparent
-public func fabs<T: FloatingPoint>(_ x: T) -> T
-  where T.Magnitude == T {
+@available(swift, deprecated: 4.2, renamed: "abs")
+public func fabs<T: FloatingPoint>(_ x: T) -> T {
   return x.magnitude
 }
 

stdlib/public/core/FloatingPoint.swift.gyb

@@ -2501,13 +2501,6 @@ where Self.RawSignificand : FixedWidthInteger,
 
 % end
 
-/// Returns the absolute value of `x`.
-@inlinable // FIXME(sil-serialize-all)
-@_transparent
-public func abs<T : FloatingPoint>(_ x: T) -> T where T.Magnitude == T {
-  return x.magnitude
-}
-
 extension FloatingPoint {
   @inlinable // FIXME(sil-serialize-all)
   @available(swift, obsoleted: 4, message: "Please use operators instead.")