build: remove linking librt for backwards compatibility

Now that we require glibc 2.17+, #17538, we can remove linking in librt
for backwards compatibility purposes. The clock_* functions from librt
were merged into glibc as part of the 2.17 release.

* The `clock_*' suite of functions (declared in <time.h>) is now available
  directly in the main C library.  Previously it was necessary to link with
  -lrt to use these functions.  This change has the effect that a
  single-threaded program that uses a function such as `clock_gettime' (and
  is not linked with -lrt) will no longer implicitly load the pthreads
  library at runtime and so will not suffer the overheads associated with
  multi-thread support in other code such as the C++ runtime library.

https://sourceware.org/ml/libc-announce/2012/msg00001.html

Note that librt is already not linked by the RISC-V and AARCH64 binaries.

build_msvc/bitcoin_config.h

@@ -218,9 +218,6 @@
 /* Define to 1 if you have the `rpcrt4' library (-lrpcrt4). */
 #define HAVE_LIBRPCRT4 1
 
-/* Define to 1 if you have the `rt' library (-lrt). */
-/* #undef HAVE_LIBRT */
-
 /* Define to 1 if you have the `shell32' library (-lshell32). */
 #define HAVE_LIBSHELL32 1