Remove obsolete --disable-elf-tls configure switch.

[rillian]

Support for disabling ELF-style thread local storage in
the standard library at configure time was removed in
pulls #30417 and #30678, in favour of a member in
the TargetOptions database. The new mentod respects
MACOSX_DEPLOYMENT_TARGET on macOS, addressing the
original use case for this configure optionl

However, those commits left the configure option itself
in place. It's no longer referenced anywhere and can
be removed.

[rust-highfive]

r? @aturon

(rust_highfive has picked a reviewer for you, use r? to override)

[rillian]

Per #32047, this switch isn't connected to anything any more. In #30678 (comment)@alexcrichton asked that the switch remain available to avoid breaking anyone's build scripts. That was over a year ago though, and in particular I know we no longer need this for Gecko, so I think it's less confusing to remove it at this point.

[aturon]

Note that @alexcrichton, in that comment, also suggested a deprecation warning (which is our usual practice before removal). AFAIK no such warning was implemented?

This is largely a policy question, so I'd like to bring in @rust-lang/compiler for their thoughts as well. (I don't personally have a strong opinion here.)

[alexcrichton]

I doubt anyone ever really used this much less even knew about it, I don't think we need much ceremony to remove it personally, it's a tiny option when building rust that's easy to remove if it even needs to be.

[aturon]

@bors: r+ rollup

[bors]

📌 Commit 19cab63 has been approved by aturon

[bors]

⌛ Testing commit 19cab63 with merge 222971f...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: aturon
Pushing 222971f to master...

[rillian]

Thanks @aturon, @alexcrichton. I didn't implement a deprecation warning because the functionality was already broken. Anyone who was setting this in a build script wasn't getting what they expected anyway.