build/check_errorf.sh: rm

This is not used and was not used for quite some time, so can be safely
removed.

If there's a need to check for capitalized error messages, we can setup
stylecheck linter with enabled ST1005 (see
https://staticcheck.io/docs/configuration/options/#checks).

I ran this checker just for fun, and it gives 20+ warnings for ST1005
only, so I'm not enabling it here.

For completeness, here's how a warning from the removed script looks
like:

> Incorrect error format in file ./client/client.go: return fmt.Errorf("Status code is not OK: %v (%s)", resp.StatusCode, resp.Status)

And here's the same warning from staticcheck:

> client/client.go:214:10: ST1005: error strings should not be capitalized (stylecheck)
> 		return fmt.Errorf("Status code is not OK: %v (%s)", resp.StatusCode, resp.Status)
> 		       ^

To me, it makes sense to not capitalize error messages (and fix those
that are capitalized), but it's up to maintainers to decide.

Signed-off-by: Kir Kolyshkin <[email protected]>

Author: kolyshkin