Makefile: remove NEEDS_LIBRT build variable

Makefile: remove NEEDS_LIBRT build variable

Commit d19e3a5b21 ("Makefile: add NEEDS_LIBRT to optionally link with
librt", 2016-07-07) introduced the NEEDS_LIBRT build variable to
disassociate the HAVE_CLOCK_GETTIME variable with the unconditional
linking of the librt library. At one time, the clock_gettime() function
was not available as part of the libc library and (on some unix systems)
required linking with librt.

Commit 52fcec75ce ("config.mak.uname: define NEEDS_LIBRT under Linux, for
now", 2016-07-10) set the NEEDS_LIBRT variable in the Linux section of
the config.mak.uname file, since Debian 7 (wheezy) was one of the few
remaining distributions, with glibc 2.13, that required linking with
librt for clock_gettime(). Note that from glibc version 2.17, this is no
longer necessary.

Note that Debian 7.0 was released on May 4th, 2013 and benefited from
long term support until May 2018 when it went end-of-life. Since that
time, Linux distributions use a more up-to-date library, for example:

    Distribution   version  end of support

    Debian 8       2.19     30th June 2020
    RHEL   8       2.28     31st May  2024 *
    Ubuntu 16.04   2.23     30th Apr  2021

* paid 'Maintenance support' ends 31st May 2029

Since it is no longer required, remove NEEDS_LIBRT from the Makefile and
config.mak.uname.

Signed-off-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>