libstdc++: Remove main() from some compile-only tests

libstdc++-v3/ChangeLog:

	* testsuite/17_intro/headers/c++1998/all_attributes.cc: Remove
	unnecessary main function.
	* testsuite/17_intro/headers/c++2011/all_attributes.cc:
	Likewise.
	* testsuite/17_intro/headers/c++2014/all_attributes.cc:
	Likewise.
	* testsuite/17_intro/headers/c++2017/all_attributes.cc:
	Likewise.
	* testsuite/17_intro/headers/c++2020/all_attributes.cc:
	Likewise.

libstdc++-v3/testsuite/17_intro/headers/c++1998/all_attributes.cc

@@ -38,8 +38,3 @@
 
 #include <bits/extc++.h>
 #include <cxxabi.h>
-
-int
-main()
-{
-}

libstdc++-v3/testsuite/17_intro/headers/c++2011/all_attributes.cc

@@ -38,8 +38,3 @@
 
 #include <bits/extc++.h>
 #include <cxxabi.h>
-
-int
-main()
-{
-}

libstdc++-v3/testsuite/17_intro/headers/c++2014/all_attributes.cc

@@ -38,8 +38,3 @@
 
 #include <bits/extc++.h>
 #include <cxxabi.h>
-
-int
-main()
-{
-}

libstdc++-v3/testsuite/17_intro/headers/c++2017/all_attributes.cc

@@ -37,8 +37,3 @@
 
 #include <bits/extc++.h>
 #include <cxxabi.h>
-
-int
-main()
-{
-}

libstdc++-v3/testsuite/17_intro/headers/c++2020/all_attributes.cc

@@ -36,8 +36,3 @@
 
 #include <bits/extc++.h>
 #include <cxxabi.h>
-
-int
-main()
-{
-}