[lldb] Remove unused posix_openpt function definition for Android (#124257)

This was for the wrapper function that was in
source/Host/android/LibcGlue.cpp. Android added
support 10+ years ago.

Author: brad0

lldb/source/Host/common/PseudoTerminal.cpp

@@ -27,10 +27,6 @@
 #include <Availability.h>
 #endif
 
-#if defined(__ANDROID__)
-int posix_openpt(int flags);
-#endif
-
 using namespace lldb_private;
 
 // PseudoTerminal constructor