[lldb] (Re)enable TestThreadBacktraceRepeat on Linux

This test is consistently passing on our Ubuntu 18.04 bot [1].

[1] https://ci.swift.org/view/LLDB/job/oss-swift-5.7-lldb-linux-ubuntu-18_04/

Author: JDevlieghere

lldb/test/API/commands/thread/backtrace/TestThreadBacktraceRepeat.py

@@ -95,9 +95,6 @@ def check_two_threads(self, result_str, thread_id_1, name_1, thread_id_2, name_2
     # the comments whether it was getting two threads to the same breakpoint that was
     # problematic, or the step-out part.  This test stops at the rendevous point so I'm
     # removing the skipIfLinux to see if we see any flakiness in just this part of the test.
-    @expectedFailureAll(
-        oslist=["linux"],
-        bugnumber="llvm.org/pr23477 Test occasionally times out on the Linux build bot")
     @expectedFailureAll(
         oslist=["freebsd"],
         bugnumber="llvm.org/pr18066 inferior does not exit")