mosh-server: Remove unused mosh_read_line function

This function is buggy.  feof(file) does not become true until after
we’ve tried to read past the last character; it should instead be
checking the getc return value for EOF (which is distinct from any
unsigned char).  Really it should just be replaced by istream
&std::getline(istream &is, string &str).  Fortunately, it’s unused so
we can just delete it.

Resolves a truncated stdio return value issue found by Coverity Scan
service.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>