Review the whole documentation in order to remove deprecated / removed features

[mapedraza]

Issue #1329

• [x] References to "plugin" (and its variant "plug-in") https://github.com/telefonicaid/iotagent-node-lib/pull/1437/commits/10aa0b41326074a7d494e25f1514cfa1eb736658
• [x] References to "bidirectional" (deprecated feature) - No references found
• [x] References to "appendMode" and "IOTA_APPEND_MODE" (and its variants "append" and "APPEND") (deprecated feature) https://github.com/telefonicaid/iotagent-node-lib/pull/1437/commits/7b50a965af6d965013c80be7421531cea9b8cac3
• [x] References to "legacy" and " expressionLanguage" (legacy expressions) https://github.com/telefonicaid/iotagent-node-lib/pull/1437/commits/eea68d83deaf855b8a3024c110a84df6dd1c2be8

[mapedraza]

Remaining plugin references:

https://www.github.com/telefonicaid/iotagent-node-lib/blob/task/doc-rework-removed-deprecated/doc/admin.md#L327-L329

removed at https://github.com/telefonicaid/iotagent-node-lib/pull/1437/commits/b3c1ed6409660c38162461766804a88cbff0bf63

[mapedraza]

Remaining append mode references:

Since only append mode now is used (thus, this flag now is removed). Probably changing every reference into the doc from "update" to append makes sense

Done here: https://github.com/telefonicaid/iotagent-node-lib/pull/1437/commits/4c0b7ce3b96dd33c11be1585f349146ec99e2fe5

[mapedraza]

Remaining plugin references:

https://www.github.com/telefonicaid/iotagent-node-lib/blob/task/doc-rework-removed-deprecated/doc/admin.md#L327-L329

This should be clarified in further steps

NTC

[mapedraza]

This PR is ready to review