QTextStreamPrivate::scan(): remove unnecessary checks

The delimiter search scan() loop detects a device or string reaching
the end of input and size limits. After that, 'false' is returned only
when no data was read at all. So, there is no a reason to test anything
except 'totalSize'.

Change-Id: Ib9cc21c0f4b7863c2dafb1bb074c5cda07387b8f
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>