Fir2Ir: simplify getCachedIrTypeParameter

I could not figure out the reason for the special case in that function.
Tests do not show anything useful.

compiler/testData/codegen/box/compileKotlinAgainstKotlin/genericSetter.kt

@@ -0,0 +1,13 @@
+// MODULE: lib
+// FILE: 1.kt
+var <T> T.prop: Int
+    get() = 33
+    set(value) {}
+
+// MODULE: main(lib)
+// FILE: 2.kt
+fun <T> setProp(t: T, v: Int) {
+    t.prop = v
+}
+
+fun box() = "OK"