Squash a Berkeley DB warning

Squash a Berkeley DB warning

In __delpair(), the assignment to next_key is not used unless DEBUG is
defined, and the debugging assert can never fail--next_realkey() will
always return n + 1 as we just checked KEY_OFF(pagep, n + 1).  Remove
the variable, the assert, and the otherwise unused next_realkey()
function.  This change eliminates a warning issued by clang 14.