Can't delete BOOTCAMP partition
I found the answer in this Apple Support Communities thread.

This is what I did:

In terminal executed diskutil list and the output was

#:                       TYPE NAME                    SIZE       IDENTIFIER
0:      GUID_partition_scheme                        *1.0 TB     disk0
1:                        EFI EFI                     209.7 MB   disk0s1
2:                  Apple_HFS Macintosh HD            542.3 GB   disk0s2
3:                 Apple_Boot Recovery HD             650.0 MB   disk0s4
4:                  Apple_HFS BOOTCAMP                449.0 MB   disk0s4
5:                 Apple_Boot                         650.0 MB   disk0s5


Then I executed the following commands: 

diskutil eraseVolume jhfs+ BC1 disk0s4
diskutil list


And the output was: 

#:                       TYPE NAME                    SIZE       IDENTIFIER
0:      GUID_partition_scheme                        *1.0 TB     disk0
1:                        EFI EFI                     209.7 MB   disk0s1
2:                  Apple_HFS Macintosh HD            542.3 GB   disk0s2
3:                 Apple_Boot Recovery HD             650.0 MB   disk0s4
4:                  Apple_HFS BC1                     449.0 MB   disk0s4


then:

diskutil eraseVolume fat32 BOOTCAMP disk0s4


After that I opened Boot Camp Assistant and I was able to restore the HD to a single partition.