doc: Drop the specific requirement on Ant 1.8.1

doc: Drop the specific requirement on Ant 1.8.1

We now use maven-antrun-plugin 1.8, which pulls in Ant 1.9.4, so
we should not need to override the dependency version anymore.

Change-Id: Ib2cec2e7f62a1a7a3aae21f87693fa1d4b6ba87f
Signed-off-by: Alexandre Montplaisir <alexmonthy@efficios.com>
Reviewed-on: https://git.eclipse.org/r/66781
Reviewed-by: Hudson CI
Reviewed-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>
Tested-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>