[lldb][NFC] Remove unused overload of File::Read

Summary: It's neither used or tested here and in swift-lldb, so let's get rid of it.

Reviewers: #lldb, davide

Reviewed By: #lldb, davide

Subscribers: davide, JDevlieghere, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D67116

llvm-svn: 370802