win_security_policy: allow removing values (resolves #40869) (#41125)

* win_security_policy: allow removing values (resolves #40869)

* Removing warning

* Adding test for remove policy setting

* Fixing string comparison

lib/ansible/modules/windows/win_security_policy.ps1

@@ -194,6 +194,8 @@ if ($will_change -eq $true) {
             if ($new_value -cne $value) {
                 Fail-Json $result "Failed to change the value for key '$key' in section '$section', the value is still $new_value"
             }
+        } elseif ([string]$value -eq "") {
+            # Value was empty, so OK if no longer in the result
         } else {
             Fail-Json $result "The key '$key' in section '$section' is not a valid key, cannot set this value"
         }