Remove strlcpy.h

Summary:
The last invocation of strlcpy.h was removed in D725. The last instace
of #include <strlcpy.h> was also removedin D730. This is unused code
that no longer serves any purpose and should be removed.

Test Plan:

make check
ninja check

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: deadalnix, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D4259