Bug: removing import doesn't lead to error

[marat-rkh]

1. Have the following code:

\import Algebra.Monoid
\import Arith.Nat

\func lemma (n : Nat) : 0 + n = n => zro-left

2. Remove the second import.

Expected: Arend shows "Cannot infer instance" error for zro-left.

Actual: no errors are shown. Re-running type checking via gutter icon doesn't fix the issue. The only way to fix it is to close the project and open it again.