Sema: remove unnecessary parameter for SwiftName handling (NFCI)

This code never actually did anything in the implementation.

`mergeDeclAttribute` is declared as `static`, and referenced exactly
once in the file: from `Sema::mergeDeclAttributes`.

`Sema::mergeDeclAttributes` sets `LocalAMK` to `AMK_None`.  If the
attribute is `DeprecatedAttr`, `UnavailableAttr`, or `AvailabilityAttr`
then the `LocalAMK` is updated.  However, because we are dealing with a
`SwiftNameDeclAttr` here, `LocalAMK` remains `AMK_None`.  This is then
passed to the function which will as a result pass the value of
`AMK_None == AMK_Override` aka `false`.  Simply propagate the value
through and erase the dead codepath.

Thanks to Aaron Ballman for flagging the use of the availability merge
kind here leading to this simplification!

Differential Revision: https://reviews.llvm.org/D88263
Reviewed By: Aaron Ballman