Go to System (should be on top panel), then Start-up Manager and set the timeout to zero.
This can also be done editing the grub file in /etc/default.
This way, the menu is not shown and Grub goes directly to your default (there's no way to avoid Grub altogether).
You can also remove entries from the Grub menu and leave only the one(s) you want (i.e. remove memtest and recovery).
Memtest can be easily removed by issuing this command in the terminal:
Recovery can also be removed by editing /etc/default/grub.
sudo chmod -x 20_memtest86+
Open the file and find the last line, which read #GRUB_DISABLE_LINUX_RECOVERY="true"
and uncomment (remove the "#")