[9.18] remove isc_bind9 variable

isc_bind9 was a global bool used to indicate whether the library was being used internally by BIND or by an external caller. external use is no longer supported, but the variable was retained for use by dyndb, which needed it only when being built without libtool. building without libtool is also no longer supported, so the variable can go away.

(cherry picked from commit 935879ed)

Backport of MR !7508 (merged)