Drop function wrapping as it is redundant for now

Drop function wrapping as it is redundant for now

As currently used in the BIND source tree, the --wrap linker option is
redundant because:

  - static builds are no longer supported,

  - there is no need to wrap around existing functions - what is
    actually required (at least for now) is to replace them altogether
    in unit tests,

  - only functions exposed by shared libraries linked into unit test
    binaries are currently being replaced.

Given the above, providing the alternative implementations of functions
to be overridden in lib/ns/tests/nstest.c is a much simpler alternative
to using the --wrap linker option.  Drop the code detecting support for
the latter from configure.ac, simplify the relevant Makefile.am, and
remove lib/ns/tests/wrap.c, updating lib/ns/tests/nstest.c accordingly
(it is harmless for unit tests which are not calling the overridden
functions).