update linux prerequisites

[bigwad]

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[miyconst]

@bigwad what is the motivation for this change? The wget and unzip are needed for the next installation steps: to download & unzip the package.

[bigwad]

The wget and unzip are needed for the next installation steps: to download & unzip the package.

You're right. The intention was to install everything with a one apt-get command (we can do that because swi-prolog exists in ubuntu package archive), but at the same time I thought about restructuring the installation section. So I mistakenly removed these two packages. I'll create an issue regarding restructuring. Then this PR would only about removing add-apt-repository step

[miyconst]

Okay, I will have to check the Ubuntu package again, because as far as I remember, it didn't work.