Drop function wrapping as it is redundant for now
As currently used in the BIND source tree, the --wrap linker option is redundant because:
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static builds are no longer supported,
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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,
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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).
Closes #1725 (closed)