[llvm-objdump] Remove bool MachOOnlyFirst from printPrivateHeaders after D155045. NFC

Mach-O can just use the global variable `FirstPrivateHeader`.
If we ever manage to remove global variables, we can add a Config
variable to Dumper. Either case, the parameter is not needed.

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D156291