Remove support for `multiple = "error"` and `multiple = "warning"` in `vec_locate_matches()`

[DavisVaughan]

Compatibility warnings are being removed in dplyr 1.2.0, so after that we will be able to remove it from vctrs too

vctrs/src/match.c

Lines 1386 to 1388 in c9e8897

	// TODO: Remove deprecated support for `multiple = "error"/"warning"`
	if (!strcmp(c_multiple, "warning")) return VCTRS_MULTIPLE_warning;
	if (!strcmp(c_multiple, "error")) return VCTRS_MULTIPLE_error;

Go through and remove VCTRS_MULTIPLE_warning and VCTRS_MULTIPLE_error and any related code paths