Remove inconsistent `AbstractFixJavadocMojo#defaultVersion` default value

The defaultVersion parameter of the javadoc:fix has an inconsistent / incorrect default value (though I am not sure what the 'correct' value would be).

Inconsistencies

• The Javadoc says "By default, it is $Id:$"
• The actual and documented (on the Mojo help) default is $Id: $Id
• The field in the code has the initial value $Id: $, with a space (using Unicode escapes)
  This value seems to have no effect because @Parameter#defaultValue overwrites the initial field value.

Maybe it would therefore be easiest to:

• Remove the "By default, it is ..." sentence from the Javadoc
  It is redundant because the Mojo help documents the @Parameter#defaultValue.
• Remove the initial field value
• Optionally remove the DEFAULT_VERSION_VALUE field and directly inline the value
• Optionally change the default to the intended default value (whatever that is)

Historical background

It seems originally the default value was supposed to be $Id$, but that was apparently causing issues with SVN, so commit 0cecfaa tried to fix this by using the field initializer and Unicode escapes instead of default-value=. But this caused the first inconsistency because the Javadoc said $Id:$ (without space) but the value was $Id: $ (with space).

Later commit 3dcd209 refactored the code to use @Parameter and added a defaultValue, but that is $Id: $Id (with duplicate "Id").