Fix covered-switch-default warning in FormatManager.

Summary:
The default case doesn't need to be here as the switch covers
all possible values. If there's a new "lazy bool" value added
in the future, the compiler would start to warn about the new
case not being covered.

Reviewers: granata.enrico, clayborg

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D13084

llvm-svn: 248365