TUNIS, Tunisia — President Habib Bourguiba today replaced his premier and designated successor with an army general who has repeatedly crushed leftist and fundamentalist uprisings in this North African Arabic nation. Bourguiba gave no reason for his abrupt firing of Rashid Sfar, who became prime minister in July, 1986. The president chose Interior Minister Gen. Zine el Abidine Ben Ali as new prime minister and secretary general of the governing Destourian Socialist Party. As premier, Ben Ali, 51, is the constitutionally designated successor to the 84-year-old Bourguiba. Bourguiba has been declared president-for-life.