Once again, no. This suggestion has all the hallmarks of “we must do something, this is something, so let's do it”.
If there was no c tag then askers would use some other tag, such as programming-languages, or algorithm, or complexity (because the homework exercise is hard, i.e. complex — don't laugh, I've seen it often). Hell, even though we have a c tag, C questions don't always use it! It's hard to evaluate because the amount of programming questions we get in a given programming language is heavily dependent on the popularity of the language among askers (mainly CS/programming students), but I don't get the impression that we get, say, more Java (java) questions than C# (no tag) questions, in proportion with the potential demand. (C# is about as popular as Java on SO, but it's less popular in programming courses, and even more so in CS courses.)
In my experience on Stack Exchange, the argument “there's a tag so this must be on-topic” doesn't come up very often, and when it comes up it's often a post-hoc justification which is obviously invalid if you spend thirty seconds reading the site's scope (e.g. justifying a Windows question on Unix & Linux because “there's a [windows] tag” — I wonder how often Home Improvement gets questions about the Windows operating system in their [windows] tag). Tags are the last thing an akser types, after the title and the body of the question; how many people do you think abandon their question when they don't see an appropriate tag, as opposed to semi-randomly picking a tag just to get the question through? We get a lot of programming homework questions
- because the askers are in a computer science curriculum;
- because askers come here after getting banned from Stack Overflow for asking too many low-quality questions.
The programming language tags serve a purpose: they characterize questions about that language. The semantics of programming languages is a domain of computer science, it is on-topic here. These tags serve a need. Even if they caused some harm, which I dispute, that would not remove our need for them. If people asking off-topic because of the presence of the tag was a widespread phenomenon, we could try renaming c to semantics-of-c or some such, but I think that the phenomenon is so negligible that it isn't worth sacrificing clear tag names.