Use the default 15-seconds TCP-Keepalive

[tahajahangir]

This reverts commit 4bf8b5a128b70cad518e5deeeeeec86315428837

TCP-Keepalives are enable by default in go since go 1.12 in this commit few months after #825 were merged.

Now the default value of 15-seconds is better and safer and we can remove this.