Remove VBLUNO51 variants from targets.json

[linlingao]

Description

Currently VBLUNO51 variants do not build. They're not Mbed enabled. Remove these targets from targets.json.

Issue request type

[ ] Question
[ ] Enhancement
[x] Bug

[ciarmcom]

Internal Jira reference: https://jira.arm.com/browse/MBOCUSTRIA-1503

[0xc0170]

cc @VNGIoTLab

[0xc0170]

@linlingao if it does not build and its been almost a month, we should consider proposing a removing the target if can't be fixed easily.

What are the build errors?

[linlingao]

@0xc0170 In the issue referenced here #6551.

[MarceloSalazar]

Closing as target won't be supported in Mbed 6 - #12775