What type of rhetoric (if any) is this?
諷刺: to satirize; to mock

暗諷: make an innuendo; indirectly mock

禮義廉:  諷刺某人「無恥」。

Calling someone 「無恥」is 明駡 (direct insult)

Calling someone「禮義廉」is 暗諷 (indirectly mock)

As for the technique of omitting part of a phrase to imply someone is lacking the omitted part -- it is called  暗示 (hinting) 

Similar example:

他這人有勇無謀 (he has courage but doesn't have any strategy) clearly stating (明示) he is not smart

他這人很有勇 (he has courage) hinting (暗示) he is not smart. 

It works because 有勇無謀 is a common phrase, therefore, the hint is strong enough that anyone who know the phrase 有勇無謀 can correctly understand what is being hinted. Same goes to 禮義廉恥 --> 禮義廉