rm compat.jl

All fixes in that file are for Julia<1.6,
while CRC requires Julia>=1.6.

Author: ToucheSir

src/ChainRulesCore.jl

@@ -17,7 +17,6 @@ export ignore_derivatives, @ignore_derivatives
 # tangents
 export Tangent, NoTangent, InplaceableThunk, Thunk, ZeroTangent, AbstractZero, AbstractThunk
 
-include("compat.jl")
 include("debug_mode.jl")
 
 include("tangent_types/abstract_tangent.jl")