Regres: Fix unix build

The `ErrTimeout` type was promoted to the OS-common .go file, but I forgot to remove it from the unix .go file.

Change-Id: Ic30675bd7bc1c5289080c45e21ce9cf19966d3e8
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/40708
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Ben Clayton <bclayton@google.com>

tests/regres/shell/shell_unix.go

diff --git a/tests/regres/shell/shell_unix.go b/tests/regres/shell/shell_unix.go
index 447098b..65fe46b 100644
--- a/tests/regres/shell/shell_unix.go
+++ b/tests/regres/shell/shell_unix.go
@@ -115,14 +115,3 @@
 		return b.Bytes(), err
 	}
 }
-
-// ErrTimeout is the error returned when a process does not finish with its
-// permitted time.
-type ErrTimeout struct {
-	process string
-	timeout time.Duration
-}
-
-func (e ErrTimeout) Error() string {
-	return fmt.Sprintf("'%v' did not return after %v", e.process, e.timeout)
-}