sig-network: remove inactive members from OWNERS_ALIASES

As a part of cleaning up inactive members (who haven't been active since
beginning of 2019) from OWNERS files, this commit removes nicksardo from
sig-network aliases in OWNERS_ALIASES.

Author: nikhita

OWNERS_ALIASES

@@ -266,7 +266,6 @@ aliases:
     - freehan
     - johnbelamaric
     - mrhohn
-    - nicksardo
     - thockin
   sig-network-reviewers:
     - andrewsykim
@@ -280,7 +279,6 @@ aliases:
     - johnbelamaric
     - m1093782566
     - mrhohn
-    - nicksardo
     - rramkumar1
     - thockin
   sig-apps-approvers: