[clang-format] Remove redundant test in style-on-command-line.cpp

Summary:
rL292562 added a fix to always format if the fallback style is set to "none".
In test/Format/style-on-command-line.cpp:19 is redundant, since -fallback-style
has a default value of LLVM set in ClangFormat.cpp:72.

@amaiorano: I believe that the rest of the test cases still cover your change in
case the fallback style is explicitly set to "none". Please, if this is not the
case, initiate a discussion.

Reviewers: ioeric, bkramer

Reviewed By: ioeric

Subscribers: cfe-commits, klimek, amaiorano

Differential Revision: https://reviews.llvm.org/D28943

llvm-svn: 292604