[core] Deleted deprecated API func srt_perfmon()

[maxsharabayko]

Deleted deprecated srt_perfmon(…) function.
Also the `[[deprecated]] C++14 keyword, when applied to a function, should be used as

[[deprecated]]
SRT_API int srt_perfmon(SRTSOCKET u, SRT_TRACEINFO * perf, int clear);

while it was used like

SRT_API int srt_perfmon(SRTSOCKET u, SRT_TRACEINFO * perf, int clear) [[deprecated]];

There is no reference to this function neither in API.md, not in API_functions.md, so nothing to clean there.

[ethouris]

"perfmon" is likely an old name. It's not in the API doc because deprecated functions are removed from the doc at the moment when they are declared deprecated. But deprecated stuff should be only removed in the next major release, I think, so I'd postpone this to 1.4.0.

AFAIK [[deprecated]] was initially planned to be applicable both in front and at the end of function declaration, but now all info I obtained has fixed it in the frontal position. In this case we might need to either have both deprecated in front, or simply leave all in the hands of compiler-specific attro. Embracing whole declaration won't be so easy because a comma inside may break the number of parameters (__VA_ARGS__ might suffice), and this will make it clumsy.

Since 4.8, AFAIR, gcc in C++11 mode supports attribute mapping through namespaced attributes, so [[gnu::deprecated]] will work also with C++11, even though the standard [[deprecated]] is available only in C++14.