Suppress method redefined warnings
Suppress method redefined warnings

Problem
===

`Bugsnag::Configuration#track_sessions` and `Bugsnag::Configuration#track_sessions=` are defined twice. Ruby warns them.

Reproduce
===

```
$ bundle exec ruby -w -Ilib -rbugsnag -e ''
/home/pocke/ghq/github.com/bugsnag/bugsnag-ruby/lib/bugsnag/configuration.rb:54: warning: method redefined; discarding old track_sessions
/home/pocke/ghq/github.com/bugsnag/bugsnag-ruby/lib/bugsnag/configuration.rb:55: warning: method redefined; discarding old track_sessions=
/home/pocke/.rbenv/versions/trunk/lib/ruby/gems/2.6.0/gems/delayed_job-4.1.5/lib/delayed/worker.rb:288: warning: method redefined; discarding old max_attempts
/home/pocke/.rbenv/versions/trunk/lib/ruby/gems/2.6.0/gems/activesupport-5.2.2/lib/active_support/core_ext/module/attribute_accessors.rb:67: warning: previous definition of max_attempts was here
/home/pocke/.rbenv/versions/trunk/lib/ruby/gems/2.6.0/gems/delayed_job-4.1.5/lib/delayed/worker.rb:292: warning: method redefined; discarding old max_run_time
/home/pocke/.rbenv/versions/trunk/lib/ruby/gems/2.6.0/gems/activesupport-5.2.2/lib/active_support/core_ext/module/attribute_accessors.rb:67: warning: previous definition of max_run_time was here
```

See the first two lines (other lines are dependency gem's warnings).

Cause
===

`track_sessions` and `track_sessions=` are defined by `attr_accessor` and `alias`. The definition by `attr_accessor` has no meaning because `alias` conceal it.

Solution
===

Remove the `attr_accessor` definition.

bugsnag/bugsnag-ruby,bugsnag/bugsnag-ruby,bugsnag/bugsnag-ruby,bugsnag/bugsnag-ruby