Fix tutorials default thumbnail (Qiskit/qiskit-metapackage#1708)

* set default qiskit logo image

* remove optimization tutorials thumbnails

---------

Co-authored-by: Luciano Bello <bel@zurich.ibm.com>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

docs/conf.py

@@ -70,16 +70,7 @@
 exclude_patterns = ['_build', '**.ipynb_checkpoints']
 
 nbsphinx_thumbnails = {
-    'tutorials/optimization/1_quadratic_program': 
-    '_static/optimization/1_quadratic_program.png',
-    'tutorials/optimization/2_converters_for_quadratic_programs': 
-    '_static/optimization/2_converters.png',
-    'tutorials/optimization/3_minimum_eigen_optimizer': 
-    '_static/optimization/3_min_eig_opt.png',
-    'tutorials/optimization/4_grover_optimizer': 
-    '_static/optimization/4_grover.png',
-    'tutorials/optimization/5_admm_optimizer': 
-    '_static/optimization/5_ADMM.png',
+    '**': '_static/no_image.png'
 }
 
 nbsphinx_prolog = """