How to remove the glitch like this?

You need to visualise the fpga as several combinatorial modules with register at its input side and register at its output side:

reg input => comb => register output

and from input pin to output pins you may have several such modules in cascade. The smaller the comb section the better fmax.

The comb section is for decisions - your work

The register section serves many purposes. The main purpose is to avois samplig the comb decisions when they are not well cooked yet(glitchy or in the way) . This is the fundamental concept of fpga design, also called synchronous i.e. synchronised to central time pulse.

Another secondary purpose of registers is memory elements.