Remove textbook tutorials (#1771)

These were removed in
Qiskit/qiskit-tutorials#1479.

Note that I messed up and should have removed them from this repo before
deleting in qiskit-tutorials. CI will be broken right now for the
metapackage due to this mistake. My bad :/

Author: Eric-Arellano

docs/tutorials.rst

@@ -71,14 +71,6 @@ Operators
 
    tutorials/operators/*
 
-Sample algorithms in Qiskit
-===========================
-
-.. nbgallery::
-   :glob:
-
-   tutorials/textbook/*
-
 .. Hiding - Indices and tables
    :ref:`genindex`
    :ref:`modindex`