configure: Drop obsolete check for the alloc_size attribute

We recently bumped our requirement for Clang to at least version 6.0.
And according to:

 https://releases.llvm.org/6.0.0/tools/clang/docs/AttributeReference.html

Clang v6.0 supports the alloc_size attribute. Thus we can drop this
check in the configure script now.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20210714072855.785566-1-thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
1 file changed