Delete duplicate definition of `LinuxMemory`

[bensymonds]

I think this was added accidentally in 5c13458dfbb0a2297d4a358fd157fdd574ba76c3. It generates the following warnings in our app:

vmstat-2.3.1/lib/vmstat/memory.rb:54: warning: method redefined; discarding old available
vmstat-2.3.1/lib/vmstat/memory.rb:54: warning: method redefined; discarding old available=
vmstat-2.3.1/lib/vmstat/memory.rb:58: warning: method redefined; discarding old available_bytes
vmstat-2.3.1/lib/vmstat/linux_memory.rb:9: warning: previous definition of available_bytes was here

[bensymonds]

@threez Anything I can do to help get this merged? Hoping it's relatively uncontroversial and can be merged and released without much trouble 🤞