readme: remove redundant meson directive
readme: remove redundant meson directive

Specifying -Dtests=enabled is redundant - tests are enabled by default.

Change-Id: I8d8b57c270bac7e4914c84781b61a34790a006ca
Signed-off-by: Brad Bishop <713d098c0be4c8fd2bf36a94cd08699466677ecd@fuzziesquirrel.com>

openbmc/pldm,openbmc/pldm,openbmc/pldm