Drop KFunction.isTailrec

The fate of this modifier seems unclear, and there doesn't seem to be any
valuable use cases for it in reflection

compiler/testData/codegen/box/reflection/modifiers/functions.kt

@@ -10,7 +10,6 @@ inline fun inline() {}
 class External { external fun external() }
 operator fun Unit.invoke() {}
 infix fun Unit.infix(unit: Unit) {}
-tailrec fun tailrec() { tailrec() }
 class Suspend { suspend fun suspend(c: Continuation<Unit>) {} }
 
 val externalGetter = Unit
@@ -25,42 +24,30 @@ fun box(): String {
     assertFalse(::inline.isExternal)
     assertFalse(::inline.isOperator)
     assertFalse(::inline.isInfix)
-    assertFalse(::inline.isTailrec)
     assertFalse(::inline.isSuspend)
 
     assertFalse(External::external.isInline)
     assertTrue(External::external.isExternal)
     assertFalse(External::external.isOperator)
     assertFalse(External::external.isInfix)
-    assertFalse(External::external.isTailrec)
     assertFalse(External::external.isSuspend)
 
     assertFalse(Unit::invoke.isInline)
     assertFalse(Unit::invoke.isExternal)
     assertTrue(Unit::invoke.isOperator)
     assertFalse(Unit::invoke.isInfix)
-    assertFalse(Unit::invoke.isTailrec)
     assertFalse(Unit::invoke.isSuspend)
 
     assertFalse(Unit::infix.isInline)
     assertFalse(Unit::infix.isExternal)
     assertFalse(Unit::infix.isOperator)
     assertTrue(Unit::infix.isInfix)
-    assertFalse(Unit::infix.isTailrec)
     assertFalse(Unit::infix.isSuspend)
 
-    assertFalse(::tailrec.isInline)
-    assertFalse(::tailrec.isExternal)
-    assertFalse(::tailrec.isOperator)
-    assertFalse(::tailrec.isInfix)
-    assertTrue(::tailrec.isTailrec)
-    assertFalse(::tailrec.isSuspend)
-
     assertFalse(Suspend::suspend.isInline)
     assertFalse(Suspend::suspend.isExternal)
     assertFalse(Suspend::suspend.isOperator)
     assertFalse(Suspend::suspend.isInfix)
-    assertFalse(Suspend::suspend.isTailrec)
     assertTrue(Suspend::suspend.isSuspend)
 
     assertTrue(::externalGetter.getter.isExternal)