Remove some tests from helix retry list

[radical]

• Remove CanSendAndReceiveUserMessagesFromMultipleConnectionsWithSameUser from retry list.
  Fixes Retry CanSendAndReceiveUserMessagesFromMultipleConnectionsWithSameUser #55954

• Remove POST_ServerAbort_ClientReceivesAbort from retry list.
  Fixes Flakey test POST_ServerAbort_ClientReceivesAbort #53294

[radical]

These tests also haven't failed in the last month:

aspnetcore/eng/test-configuration.json

Lines 7 to 9 in c7fa381

	{"testName": {"contains": "MaxRequestBufferSizeTests.LargeUpload" }}, // tries to manipulate the state of the server's buffer
	{"testName": {"contains": "CertificateChangedOnDisk"}}, // depends on FS event timing
	{"testName": {"contains": "CertificateChangedOnDisk_Symlink"}}, // depends on FS event timing

@wtgodbe can these be removed, or are they expected to inherently fail some times?