Remove deprecated equals error

[casey]

Currently, the parser contains code that checks for = in contexts where it used to be allowed, namely variables and aliases, and produces an error.

This prevents writing recipes named alias and export with a parameter with a default value:

export a="b":
alias a="b":

This error was introduced in 0.9.0, released on March 28th, 2021. After a grace period, the error can be removed to allow the above recipes to work. I'm thinking six months, so October 1st, 2021.