The backtick is called grave
If the backtick is not typed on your first keyboard button press, but on the second it is a dead grave.
So, find the backtick button keycode for your layout:
xmodmap -pke | grep "grave"
which returns (for my german keyboard layout):
keycode 21 = dead_acute dead_grave dead_acute dead_grave
keycode 49 = dead_circumflex degree asciicircum degree U2032 U2033 notsign notsign grave asciitilde
keycode 51 = numbersign apostrophe numbersign apostrophe rightsinglequotemark dead_breve rightsinglequotemark grave backslash bar
The first entry (keycode 21) is the correct one (the one I physically use), with the acute (forwardtick?) and grave (backtick) symbols.
Both have a dead_ prefix, meaning they wait for another keypress.
So all you have to do, is to change the symbols for this keycode. I will keep the dead behaviour for acute (to write Café), but not for grave, so:
xmodmap -e 'keycode 21 = dead_acute grave'
Note that your keycode may be different (e.g., 49 on uk/us keyboards). If you want to have consistent behaviour, remove the dead prefix from acute aswell.
If you came here to avoid typing any tick/backtick key twice, just select a keyboard layout saying "no dead keys".