[FIR JS] Remove weird source picking in FirJsExternalChecker

Otherwise, these tests fail due to
misplaced diagnostics in LT:
- `testExtensionFunctionArgumentOrReturnType`
- `testInlineClassAsParameterOrReturnType_allowed_kt`
- `testInlineClassAsParameterOrReturnType_kt`

compiler/fir/checkers/checkers.js/src/org/jetbrains/kotlin/fir/analysis/js/checkers/declaration/FirJsExternalChecker.kt

@@ -88,12 +88,7 @@ object FirJsExternalChecker : FirWebCommonExternalChecker() {
             }
 
             for (parameter in declaration.valueParameters) {
-                val ktParam = if (parameter.source?.psi is KtParameter) {
+                val ktParam = parameter.source
-                    parameter.source
-                } else {
-                    declaration.source
-                }
-
                 if (ktParam?.allowsReporting != true) {
                     continue
                 }