remove garbage tabs

testfile.xlsx has 2x2 cells, but git-xlsx-textconv outputs 4x2, garbage tabs exists.

```
% git-xlsx-textconv $GOPATH/github.com/tokuhirom/git-xlsx-textconv/testfile.xlsx | cat -A
[Tabelle1] ^I^IFoo^IBar$
[Tabelle1] ^I^IBaz^IQuuk$
```

This commit removes garbage tabs.

```
% go get github.com/minimum2scp/git-xlsx-textconv
% cd $GOPATH/src/github.com/minimum2scp/git-xlsx-textconv
% git checkout -t fix/delete-garbage-tabs
% go run ./git-xlsx-textconv.go ./testfile.xlsx | cat -A
[Tabelle1] Foo^IBar$
[Tabelle1] Baz^IQuuk$
```