libc6-dev no longer required for C#

tools/dockerfile/distribtest/csharp_centos7_x64/Dockerfile

@@ -36,4 +36,3 @@ RUN yum install -y mono
 RUN yum install -y git
 RUN yum install -y unzip
 RUN yum install -y nuget
-

tools/dockerfile/distribtest/csharp_jessie_x64/Dockerfile

@@ -41,7 +41,3 @@ RUN apt-get update && apt-get -y dist-upgrade && apt-get install -y \
     nuget
 
 RUN apt-get update && apt-get install -y git unzip
-
-# TODO(jtattermusch): Currently P/Invoke to dlopen will fail without libc6-dev
-# being installed. Figure out how to workaround this.
-RUN apt-get update && apt-get install -y libc6-dev

tools/dockerfile/distribtest/csharp_jessie_x86/Dockerfile

@@ -41,7 +41,3 @@ RUN apt-get update && apt-get -y dist-upgrade && apt-get install -y \
     nuget
 
 RUN apt-get update && apt-get install -y git unzip
-
-# TODO(jtattermusch): Currently P/Invoke to dlopen will fail without libc6-dev
-# being installed. Figure out how to workaround this.
-RUN apt-get update && apt-get install -y libc6-dev

tools/dockerfile/distribtest/csharp_wheezy_x64/Dockerfile

@@ -30,7 +30,3 @@
 FROM mono:4.2.2.30
 
 RUN apt-get update && apt-get install -y git unzip
-
-# TODO(jtattermusch): Currently P/Invoke to dlopen will fail without libc6-dev
-# being installed. Figure out how to workaround this.
-RUN apt-get update && apt-get install -y libc6-dev