(library-wide) Remove inline from variable templates

[CaseyCarter]

Removes extraneous inline from the definitions of variable templates in [format.syn], [format.tuple], [time.format], [execution.syn], and [exec.util.cmplsig].

[jensmaurer]

Seems straightforward.

@jwakely , any thoughts?

[jwakely]

I think making them inline was made unnecessary by CWG 2387, right? If so, I'm fine with dropping inline now.

GCC only implemented that three weeks ago, which might have contributed to my confusion about whether these inline keywords were needed.

[jensmaurer]

We've had earlier incarnations of inline removal for these cases, e.g. 9568c66 and d2ba9e3 and 9ac5555 .