dtrace: remove x86 non-EARLY_AP_STARTUP support
After 792655abd64c EARLY_AP_STARTUP is mandatory for x86.
Reviewed by: markj
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D42139
(cherry picked from commit e49c7cd677df0b8691c828b55919ef37d8aad219)