Fix readthedocs build by removing duplicate entry

The pip 2020 dependency resolver considers two dependencies from the
same VCS url to be two incompatible dependencies.
diff --git a/requirements.txt b/requirements.txt
index 0bb0044..4d97877 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,4 +1,3 @@
--e git+https://github.com/apache/airavata.git@5a7d80193317cd73b4d33f3f3f2745f1a252c32c#egg=airavata_python_sdk&subdirectory=airavata-api/airavata-client-sdks/airavata-python-sdk
 bcrypt==3.1.7
 cffi==1.14.1
 cryptography==3.0