[api-minor] Remove the disableCombineTextItems option

*Please note:* This parameter has never been used within the PDF.js library/viewer itself, and it was only ever added for backwards compatibility reasons.

This parameter was added in PR 7475, over six years ago, to try and optionally maintain the previous *default* text-extraction behaviour.
However as part of the general text-extraction improvements in PR 13257, almost two years ago, the `disableCombineTextItems` functionality was accidentally "broken" in various ways. Note how the only (very basic) unit-test was updated in a way that doesn't really make sense, since generally speaking you'd expect that using the option should result in *more* (or at least the same number of) text-items. Furthermore there's also the recent issue 16209, where the option causes almost all textContent to be concatenated together.

Hence this patch proposes that we simply remove the `disableCombineTextItems` option since it's essentially unused/untested functionality, as evident from the fact that it took almost two years for someone to notice that it's broken.

src/core/annotation.js

@@ -1010,7 +1010,6 @@ class Annotation {
       task,
       resources,
       includeMarkedContent: true,
-      combineTextItems: true,
       sink,
       viewBox,
     });