Remove stanc flags munging for `cpp_options$stan_opencl`

[WardBrian]

This code:

cmdstanr/R/model.R

Lines 538 to 540 in edccf2d

	if (isTRUE(cpp_options$stan_opencl)) {
	stanc_options[["use-opencl"]] <- TRUE
	}

Is redundant with the logic already in the cmdstan makefiles:
https://github.com/stan-dev/cmdstan/blob/b2d14a30df8a843115aae9045ff5c01a031561e7/make/program#L59-L62

And is generating invalid calls to stanc3, see https://discourse.mc-stan.org/t/cannot-complile-and-then-use-gpu/40315

[jgabry]

After chatting with @WardBrian, we're going to hold off on changing this in cmdstanr and probably fix it in the compiler via stan-dev/stanc3#1541 or maybe in the cmdstan makefiles.