Remove redundant backticks: fix false positive for multiple underscores

#KT-37496 Fixed

idea/testData/inspectionsLocal/removeRedundantBackticks/underscores.kt

@@ -0,0 +1,4 @@
+// PROBLEM: none
+fun test() {
+    val <caret>`__` = ""
+}

idea/testData/inspectionsLocal/removeRedundantBackticks/underscores2.kt

@@ -0,0 +1,4 @@
+// PROBLEM: none
+fun test() {
+    val <caret>`__________` = ""
+}