Remove extra -headerpad linker argument

[adamnovak]

Changelog Entry

To be copied to the draft changelog by merger:

• vg mac builds no longer pass -headerpad without an argument to the linker.

Description

According to the Clang 15 man page, -headerpad needs a size argument:

-headerpad size
        Specifies the minimum space for future expansion of the load commands.  Only useful
        if intend to run install_name_tool to alter the load commands later. Size is a
        hexadecimal number.

-headerpad_max_install_names
        Automatically adds space for future expansion of load commands such that all paths
        could expand to MAXPATHLEN.  Only useful if intend to run install_name_tool to alter
        the load commands later.

I don't think it does anything we need that -headerpad_max_install_names doesn't do. And it is causing build failures for some people (see https://www.biostars.org/p/9598655/) because it doesn't have an argument.

[adamnovak]

This might be needed to actually fix #4257; we never worked out how to reproduce it, so it's hard to tell.