chore(ci): drop the temporary macOS crash-diagnostic workflow

diag-ut-macos.yml was a one-shot forensics harness for the boost-ext.ut macOS
SIGSEGV — lldb disassembly, __init_offsets symbolization, a dozen relink
variants — triggered on push to this branch. The crash is diagnosed
(mcpp-community/mcpp#336, fixed in mcpp 2026.8.3.1), so the harness has done
its job. On main it would never fire and only burn macos-15 minutes on manual
dispatch.

Co-authored-by: SPeak Agent <248744407+speak-agent@users.noreply.github.com>

Author: Sunrisepeak