[MergeFuncs] Remove incorrect attribute copying

Fix for https://bugs.llvm.org/show_bug.cgi?id=44236. This code was
originally introduced in rG36512330.
However, the attribute copying was done in the wrong place (in general
call replacement, not thunk generation) and a proper fix was
implemented in D12581.

Previously this code was just unnecessary but harmless (because
FunctionComparator ensured that the attributes of the two functions
are exactly the same), but since byval was changed to accept a type
this copying is actively wrong and may result in malformed IR.

Differential Revision: https://reviews.llvm.org/D71173