[Libreoffice] rtl::O[U]StringBuffer now has a "remove" method

[Caolán McNamara]

I've added a "remove" method to the O[U]StringBuffers to make it easy to
remove text from them without painful assembly of a new one by copying
segments out of an old one.

These StringBuffers were originally modelled after the Java equivalents.
Since the sal ones were written Java added "delete" methods to their
string buffers but delete is a reserved c++ keyword, so I called them
"remove" here.

It takes start position and end position in usual half-open [x,y) style,
not start position and length though this is clearly the same for an x
of 0

C.