[Docs] Remove llvm-objdump man page (#75759)

This patch removes the explicit llvm-objdump man page. By enabling
sphinx man page output with `-DLLVM_ENABLE_SPHINX=ON` and
`-DSPHINX_OUTPUT_MAN=ON`, we can generate man pages for all the llvm
binary utilities from the restructured text documentation. Having an
additional man page upstream increases fragementation and maintenance.