docs: update uninstall.md to include removal of bash.bashrc.backup-before-nix file

[BBBmau]

Motivation

Was following the uninstall docs and still couldn't get the install to work until i realized i needed to remove the actual bash.bashrc.backup-before-nix file completely despite it being empty. I've added this in the final note of the docs in the event that others run into this issue also.

Context

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[abathur]

Moving this file out of the way is already documented in step 1:

nix/doc/manual/src/installation/uninstall.md

Line 70 in 641b0bd

sudo mv /etc/bash.bashrc.backup-before-nix /etc/bash.bashrc

[BBBmau]

@abathur this only moves the contents of /etc/bash.bashrc.backup-before-nix into /etc/bash.bashrc this does not delete the file once moved which is where my issue comes from since I had to explicitly do a deletion of /etc/bash.bashrc.backup-before-nix even after doing all the steps. I still propose that we include this.

[fricklerhandwerk]

That doesn't sound right. mv literally moves the file, the original file thus won't exist afterwards.