Remove redundant spaces in _outA_Expr() output

Remove redundant spaces in _outA_Expr() output

Since WRITE_NODE_FIELD() output always starts with a space, we don't
need to go out of our way to print another space right before it.

This change is only for visual appearance; the tokenizer on the
reading side would read it the same way (but there is no read support
for A_Expr at this time anyway).