Update MO requirements to allow TF1.15 if already installed (#10673)

* Update MO requirements to allow TF1.15 if already installed

* Removing pyhton version check as redundant

* Updating requirements.txt as well

tools/mo/requirements.txt

@@ -1,5 +1,4 @@
-tensorflow~=2.5.0; python_version <= "3.6"
-tensorflow~=2.5.3; python_version > "3.6"
+tensorflow>=1.15.5,<2.6
 mxnet~=1.2.0; sys_platform == 'win32'
 mxnet~=1.7.0.post2; sys_platform != 'win32'
 networkx~=2.5; python_version <= "3.6"

tools/mo/requirements_tf.txt

@@ -1,5 +1,4 @@
-tensorflow~=2.5.0; python_version <= "3.6"
-tensorflow~=2.5.3; python_version > "3.6"
+tensorflow>=1.15.5,<2.6
 networkx~=2.5; python_version <= "3.6"
 networkx~=2.6; python_version > "3.6"
 numpy>=1.16.6,<1.20