ci: drop unused BUILD_TYPE, add useful RELEASE param

It appears this was some leftover from ancient times and wasn't being used.

On the other hand the `RELEASE` environment variable controls if Nim builds
of the client binary include debug symbols:
ba7a6d5d34/Makefile (L177-L184)

The shorthand `?:` symbol means that if this is changed for a given job it stays changed.

Signed-off-by: Jakub Sokołowski <jakub@status.im>

ci/Jenkinsfile.linux

@@ -11,10 +11,10 @@ pipeline {
   }
 
   parameters {
-    string(
-      name: 'BUILD_TYPE',
-      description: 'Specify build type. Values: pr / nightly / release',
-      defaultValue: 'pr',
+    booleanParam(
+      name: 'RELEASE',
+      description: 'Decides whether binaries are built with debug symbols.',
+      defaultValue: params.RELEASE ?: false
     )
   }
 

ci/Jenkinsfile.macos

@@ -6,10 +6,10 @@ pipeline {
   }
 
   parameters {
-    string(
-      name: 'BUILD_TYPE',
-      description: 'Specify build type. Values: pr / nightly / release',
-      defaultValue: 'pr',
+    booleanParam(
+      name: 'RELEASE',
+      description: 'Decides whether binaries are built with debug symbols.',
+      defaultValue: params.RELEASE ?: false
     )
   }
 

ci/Jenkinsfile.windows

@@ -4,10 +4,10 @@ pipeline {
   agent { label 'windows' }
 
   parameters {
-    string(
-      name: 'BUILD_TYPE',
-      description: 'Specify build type. Values: pr / nightly / release',
-      defaultValue: 'pr',
+    booleanParam(
+      name: 'RELEASE',
+      description: 'Decides whether binaries are built with debug symbols.',
+      defaultValue: params.RELEASE ?: false
     )
   }