Remove unused parameter from TSMS IPC

TextServicesManager always sets null to the "locale" parameter in
ITextServicesManager#getCurrentSpellCheckerSubtype then
TextServicesManagerService always ignores that parameter.  It can be
safely removed.

Bug: 110275412
Test: Manually verified that spell checker still works
Change-Id: I5d85089899b0bd077f69971d77095859086a8d52