MIPS/BFD: Remove redundant "want64=true" settings

Clean up after commit 29c108c96106 ("MIPS: Support `-gnuabi64' target
triplet suffix for 64-bit Linux targets") and discard individual MIPS
"want64=true" settings, the use of which has been superseded by commit
42429eacb42f ("Require a 64-bit bfd_vma for MIPS ELF") back in 2013[1].

References:

[1] <https://inbox.sourceware.org/binutils/87mwqg4mfn.fsf@talisman.default/>