Temporarily turn off MK_PKGBOOTSTRAP evaluation to fix build.

Discussed with:	bapt
Sponsored by:	The FreeBSD Foundation
MFC 238010:
  Add PKGBOOTSTRAP to __DEFAULT_YES_OPTIONS to fix build.

Sponsored by:	The FreeBSD Foundation
Revert r263047 now that the missing commit was merged for pkg(7)
updates.

Sponsored by:	The FreeBSD Foundation
MFC r237997, r238018, r238023, r257440, r258924:
  Add WITHOUT_PKGBOOTSTRAP options file.

Sponsored by:	The FreeBSD Foundation