fdo#70144 Don't reset Document's modified flag when inserting new table.

It is unclear why the modified flag should ever be blindly overwritten hence
we no longer touch the modified flag here. (This change appears to have no
undesired side-effects.)

Change-Id: I246eb7d84249794fa4d52fcd9b008d8340bbc82d
Reviewed-on: https://gerrit.libreoffice.org/6632
Reviewed-by: Lionel Elie Mamane <lionel@mamane.lu>
Tested-by: Lionel Elie Mamane <lionel@mamane.lu>
(cherry picked from commit 56d4f4b7b97eaf212a3bc6bdb1da767106e7224a)