configure: Do not use default_feature for EXESUF

Commit "c87ea116 configure: add --without-default-features" use
default_feature to set default values for configure option. This value
is used for EXESUF too.

However, EXESUF is not option to be tested, it is just append to any
binary name so using --without-default-features set EXESUF to "n"o and
all binaries using it has form <name>no (e.g. qemu-imgno).

This is not expected behavior as disabling features should not cause
generating different binary names.

Reverting back to setting EXESUF to empty value unless needed otherwise.

Signed-off-by: Miroslav Rezanina <mrezanin@redhat.com>
Message-Id: <20210331081845.105089-1-mrezanin@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>