moved 'mergeTopMarks' out of 'WriteCollectedRunProperties'

The reason for this - is that the function 'WriteCollectedRunProperties'
needs to be used by the redline exporter (to export collected 'redlined'
properties) - and the 'mergeTopMarks' is not needed there.

Change-Id: I10063b2f28adab53949b4d9de8acd0dab66553b0
Reviewed-on: https://gerrit.libreoffice.org/6899
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>