refactor: remove paging types from apps endpoint

- remove `PagingRequest`, `PagingOption`, and `Page` struct from
`apps.go`
- remove unused `strconv` import

pagingRequest.go

@@ -0,0 +1,17 @@
+package onspring
+
+import "strconv"
+
+type PagingRequest struct {
+	pageSize   int
+	pageNumber int
+}
+
+func (pr *PagingRequest) ToParams() map[string]string {
+	return map[string]string{
+		"pageSize":   strconv.Itoa(pr.pageNumber),
+		"pageNumber": strconv.Itoa(pr.pageSize),
+	}
+}
+
+type PagingOption func(*PagingRequest)