AccessibleEditableTextPara::GetParagraphCount no longer restricted

...to sal_uInt16, after 2af1f569 "resolved
fdo#35756 import more than 64k HTML table cells".  Some checks against USHRT_MAX
had not been removed at all, while others had been changed to useless checks
against SAL_MAX_INT32.

Change-Id: If21d10426ee392ba3f0586dde766e3f7da99d214
Reviewed-on: https://gerrit.libreoffice.org/48769Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>