Keep the database schema at version 1 (closes #61)

sfdupes is pre-1.0, with no installed base and no databases anywhere,
so the schema is changed in place and its version stays 1.
schemaVersion goes back to 1; the six-column files table, content
included, is the version 1 schema. The check that stops on a database
with any other version stays. README.md and TODO.md no longer describe
a version 2 or rejecting and rescanning version 1 databases. The
main.go package comment still described 1024-byte end windows and said
full file contents are never read; it now describes the hashes the
code computes.

Model: opus-5-5

db.go

@@ -25,11 +25,8 @@ const defaultDatabasePath = "/var/lib/sfdupes/db.sqlite"
 const databaseEnv = "SFDUPES_DATABASE"
 
 // schemaVersion is the database schema version this build reads and
-// writes, stored in PRAGMA user_version. Version 2 adds the content
-// column and the head/tail/content signature (replacing the version 1
-// 1 KiB end windows), so a version 1 database is rejected and must be
-// rescanned.
-const schemaVersion = 2
+// writes, stored in PRAGMA user_version.
+const schemaVersion = 1
 
 // dbDirPerm is the mode for a database parent directory created by
 // scan.