Kapt: remove old JVM backend test data (2/2)

In this commit, .ir.txt expectation test data files for the JVM IR
backend are renamed to .txt. This is done as two separate commits so
that Git would recognize that these files were logically moved, not that
the contents of the original .txt files were changed.

 #KT-64680

plugins/kapt3/kapt3-compiler/testData/converter/multifileClass.txt

@@ -0,0 +1,41 @@
+package test;
+
+/**
+ * multi-file class {
+ *   // test/M1__AKt
+ *   // test/M1__BKt
+ * }
+ */
+@kotlin.Metadata()
+public final class M1 {
+
+    public M1() {
+        super();
+    }
+
+    public static final void bar() {
+    }
+
+    public static final void foo() {
+    }
+}
+
+////////////////////
+
+package test;
+
+/**
+ * multi-file class {
+ *   // test/M2__CKt
+ * }
+ */
+@kotlin.Metadata()
+public final class M2 {
+
+    public M2() {
+        super();
+    }
+
+    public static final void baz() {
+    }
+}