Remove unused import statements in generated Java files

[yutaro-sakamoto]

All generated Java files contain the following import statements.

import java.io.UnsupportedEncodingException;
import jp.osscons.opensourcecobol.libcobj.*;
import jp.osscons.opensourcecobol.libcobj.common.*;
import jp.osscons.opensourcecobol.libcobj.data.*;
import jp.osscons.opensourcecobol.libcobj.exceptions.*;
import jp.osscons.opensourcecobol.libcobj.termio.*;
import jp.osscons.opensourcecobol.libcobj.call.*;
import jp.osscons.opensourcecobol.libcobj.file.*;
import jp.osscons.opensourcecobol.libcobj.ui.*;
import java.util.Optional;
import java.lang.NullPointerException;
import java.lang.IndexOutOfBoundsException;

Some programs do not require some import statements.
For example, COBOL programs that do not use file I/O do not require import jp.osscons.opensourcecobol.libcobj.file.*;.