Remove deprecated text attribute from <body> in calc export to html

Text attribute was used to set text color to black. Should be in css and most browsers default to black.

Change-Id: I966057ca97e2622cc51727ab64796fe36a33d6c4
Reviewed-on: https://gerrit.libreoffice.org/8044
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>