If a runoff candidate withdraws, the remaining candidate is considered to be elected and the runoff election for that office is not held.
Tex. Elec. Code § 145.095
EFFECT OF WITHDRAWAL FROM RUNOFF
Acts 1985, 69th Leg., ch. 211, Sec. 1, eff
Official source: Texas Constitution and Statutes. Reproduced from public-domain Texas statutes; confirm against the official source for the current text. Not legal advice.