You can remove an attribute value by setting it to nothing (a longer explanation is below)
$MyAttr=;
Note the semicolon is not quoted, even for attribute type like strings. You can also use a empty string for string types:
$MyString="";
Only the first of the syntaxes above also allows attributes that use prototype or preference settings to re-inherit these again. So, do not use an empty string as a reset value unless that is your definite need. See about resetting inheritance for more detail.
To explain the syntax further, you are effectively doing this to 'reset' an attribute to default inherited value:
$MyAttr=
…and then explicitly then adding a semi-colon to close the expression, thus…
$MyAttr=;
Note that the semi-colon is not part of the reset expression. So the last-but-one code above, even if used as a rule on its own, will work but is not recommended. The additional semi-colon makes sure Tinderbox does not have to guess your intent and also avoids you having to later remember to add a semi-colon if you add more code on the end of the original unclosed expression.