The 26-year-old Wakefield prop tested positive for cocaine after the Betfred Super League win at Widnes on July 14 and was subsequently suspended by Trinity.
Scotland RL chairman Keith Hogg said: “We totally support the RFL anti-doping regulations and the action undertaken by Wakefield Trinity.
“We will be withdrawing Adam Walker from our extended World Cup squad.”
Head coach Steve McCormack has added NRL pair James Bell and Campbell Graham to what is now a 37-man squad following Walker’s removal.
Bell, of New Zealand Warriors, and South Sydney Rabbitohs’ winger Graham both made their NRL debuts last week.
“We are always watching a number of Scottish qualified players at clubs in the UK, Australia and New Zealand,” McCormack said.
“We’ve known about them since last year and have been really pleased to see the progress that James and Campbell have been making. It was great to see them make their NRL debuts within a few days of each other.”