Remove TCP transmit zerocopy shutdown when removing TCP endpoint from pollset. (#25782)
Remove TCP transmit zerocopy shutdown when removing TCP endpoint from pollset. (#25782)

Previously, we disabled TCP transmit zerocopy and waited for all pending
completions when removing a TCP endpoint from a pollset. However, certain
transports may do this and then re-add the endpoint to a pollset, in which case
zerocopy is no longer active.

Instead, here we only shutdown zerocopy if the socket is being torn down.
vjpai/grpc,vjpai/grpc,ctiller/grpc,grpc/grpc,donnadionne/grpc,vjpai/grpc,ejona86/grpc,ctiller/grpc,nicolasnoble/grpc,jtattermusch/grpc,ejona86/grpc,stanley-cheung/grpc,stanley-cheung/grpc,donnadionne/grpc,ctiller/grpc,jtattermusch/grpc,nicolasnoble/grpc,ctiller/grpc,grpc/grpc,nicolasnoble/grpc,stanley-cheung/grpc,ctiller/grpc,vjpai/grpc,donnadionne/grpc,jtattermusch/grpc,vjpai/grpc,donnadionne/grpc,jtattermusch/grpc,jtattermusch/grpc,donnadionne/grpc,stanley-cheung/grpc,grpc/grpc,donnadionne/grpc,ctiller/grpc,donnadionne/grpc,ejona86/grpc,ctiller/grpc,ejona86/grpc,jtattermusch/grpc,stanley-cheung/grpc,stanley-cheung/grpc,ejona86/grpc,vjpai/grpc,jtattermusch/grpc,grpc/grpc,jtattermusch/grpc,ejona86/grpc,ejona86/grpc,ctiller/grpc,vjpai/grpc,donnadionne/grpc,nicolasnoble/grpc,grpc/grpc,nicolasnoble/grpc,stanley-cheung/grpc,stanley-cheung/grpc,grpc/grpc,ejona86/grpc,stanley-cheung/grpc,ejona86/grpc,stanley-cheung/grpc,ctiller/grpc,nicolasnoble/grpc,donnadionne/grpc,jtattermusch/grpc,ctiller/grpc,grpc/grpc,donnadionne/grpc,vjpai/grpc,grpc/grpc,ctiller/grpc,jtattermusch/grpc,vjpai/grpc,vjpai/grpc,nicolasnoble/grpc,jtattermusch/grpc,nicolasnoble/grpc,jtattermusch/grpc,ejona86/grpc,ejona86/grpc,vjpai/grpc,nicolasnoble/grpc,nicolasnoble/grpc,grpc/grpc,grpc/grpc,nicolasnoble/grpc,grpc/grpc,stanley-cheung/grpc,stanley-cheung/grpc,nicolasnoble/grpc,ctiller/grpc,donnadionne/grpc,grpc/grpc,ejona86/grpc,donnadionne/grpc,vjpai/grpc