Remove db_spec.md

doc/db_spec.md

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-# Database specification
-
-## Lesson IDs
-
-`lesson_id` and `g*` equivalents of lessons.
-
-- Academic Writing
-    - 'aw'
-- Biology
-    - 'bi'
-- Business Management
-    - 'bm'
-- Chemistry
-    - 'ch'
-- Computer Science
-    - 'cs'
-- Economics
-    - 'ec'
-- English A
-    - 'ena'
-- English B
-    - 'enb'
-- French
-    - 'fr'
-- Geography
-    - 'ge'
-- German
-    - 'de'
-- Guidance
-    - 'gu'
-- Mathematical Studies
-    - 'mst'
-- Mathematics HL
-    - 'mhl'
-- Mathematics SL
-    - 'msl'
-- Polish A
-    - 'pl'
-- Psychology
-    - 'ps'
-- Self-Taught Language A
-    - 'st'
-- Spanish
-    - 'es'
-- Theory of Knowledge
-    - 'tok'
-- Turkish A
-    - 'tr'
-- Visual Arts
-    - 'va'